This relates generally to image sensors and, more specifically, to image sensors having photodiode regions implanted from both sides of a semiconductor substrate.
Modern electronic devices such cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) include a two-dimensional array of image sensing pixels. Each pixel includes a photosensor such as a photodiode that receives incident photons (light) and converts the photons into electrical charges. The photodiodes in the array are implanted in a silicon substrate.
In conventional image sensors, the photodiodes are implanted in the silicon substrate through a single surface of the substrate using pattern-implant equipment. After implantation, the silicon substrate is thermally heated to activate the implant dopants. In general, it is desirable to implant the photodiodes at greater depths below the surface of the substrate to increase the light collection efficiency of the sensor. However, implanting the photodiodes through a single surface of the substrate to great depths requires high energy. High energy implants require very thick resist or other dense masks to prevent ions leaking through the masks. This is exacerbated with finer dimensions. In addition, if the mask is excessively thick, to accommodate deep implants, shadowing effects will occur. Such limitations on the depth of the photodiode implants undesirably limit the light collection efficiency of the image sensor.
It would therefore be desirable to be able to provide improved image sensors.